Such a radio telecommunication system is already known in the art, e.g. from the article "Improving the Performance of a Slotted ALOHA Packet Radio Network with an Adaptive Array" by A. Ward and R. T. Compton, published in IEEE TRANSACTIONS ON COMMUNICATIONS, VOL.40, NO.2, February 1992, pages 292 to 300. Therein, the data packets are transmitted according to the well known Slotted-ALOHA (S-ALOHA) scheme which is for instance described in the book "DIGITAL COMMUNICATIONS--Fundamentals and Applications" of B. Sklar, edited by Prentice-Hall International, Inc., 1988, and more particularly in Chapter 9, pages 475 to 505, thereof. A problem with the S-ALOHA is that when two or more emitter stations are simultaneously transmitting data packets in a same channel, a collision occurs so that the receiver station is no longer able to distinguish between the emitter stations from which the data packets are received. As a consequence, all these data packets are then generally rejected and the emitter stations have to transmit them again. Obviously this leads to an important increase of the traffic load on the radio link.
This problem with the S-ALOHA scheme is partially solved in the system according to the above mentioned article by the use of an adaptive array of sensors or antennas instead of the generally used omnidirectional antenna. The performance of the radio telecommunication system is thereby improved as will be explained below.
When the emitter stations are not transmitting data packets, the array of sensors operates as an omnidirectional antenna and is thus adapted to capture any data packet coming from any direction, i.e. from any of the emitter stations. After having detected the beginning, i.e. a training sequence of bits, of such a data packet transmitted by an emitting station, the receiver station points its antenna array in the direction of this transmitting emitter station, thereby ignoring then other data packet transmitted by other emitting stations. However, such pointing in the correct direction is only possible if two (or more) data packets simultaneously transmitted are separated geographically and in time, i.e. are incoming from different directions and mutually shifted over a certain number of bits. In this case, only the firstly arriving data packet is accepted by the receiver station, while any following data packet transmitted by another emitter station in the same channel is ignored.
The improvement proposed in the above article increases significantly the capacity of some radio telecommunication applications since, in case of collision, the colliding data packets are not systematically rejected all, but the first one of them is generally accepted. Moreover, a data packet which is not accepted during a first predetermined channel because of a collision will generally be accepted after a few transmission retrials in other channels. This means that, even if as in the above known radio telecommunication system all the channels of a time frame of the radio link may be used to transmit and receive data packets, the collisions still lead to a traffic overload.
This remaining drawback of traffic increase is even more significant in case of a mobile radio telecommunication system wherein generally only a single predetermined channel of each time frame of the radio link may be used by the mobile emitter stations to transmit a so-called "call set-up packet" to the fixed base receiver station in order to establish a communication therewith. In such a mobile communication system the other channels of the radio link either are used for transmitting signalling information or are individually allocated by the base station to distinct mobile stations for communication therewith by means of "user information packets".
The problem just described for instance occurs in the pan-European "Global System for Mobile communications" GSM which is a digital cellular network wherein each cell includes one base station and a plurality of mobile stations moving in the cell. Since the mobile stations are allowed to transmit call set-up packets to the base station only in a single predetermined channel of a radio link, i.e. at a predetermined frequency of this radio link and during a predetermined time slot thereof, there is a relatively high risk of collision of such packets transmitted by the mobile stations and simultaneously attempting to access the base station.
A possible solution to decrease the possibility of collision is to increase the number of predetermined channels in which the mobile stations are allowed to transmit call set-up packets. However, this solution gives rise to a low efficiency of use of the resources of the base station because these channels are then not available for other purposes such as signalling or normal communication traffic, as mentioned above.
Even if the improvement disclosed in the above article would be applied to the particular case of a mobile telecommunication system, such as GSM, the traffic load of the base station would still be too high because of the necessary re-transmission of colliding packets which were ignored by the base station, i.e. which did not arrive first in the predetermined channel. Moreover, in the known system only some colliding call set-up packets may be recovered, while colliding user information packets may not be saved. This is due to the fact that the duration of a call set-up packet is shorter than the duration of the predetermined channel whereby, as mentioned above, the first one of several call set-up packets received in a same predetermined channel but sufficiently shifted in time may be recognized by the receiver station. On the other hand, the duration of a user information packet corresponds substantially to the duration of the channel whereby the time shift of two or more user information packets received in a same channel is never sufficient for the receiver station to separate them.